In a cellular wireless system, a service area is divided into a number of coverage zones generally referred to as cells. Each cell may be further subdivided into a number of sectors. Wireless terminals in a cell communicate with the base station that serves the cell. Wireless terminals may include a wide range of mobile devices including, e.g., cell phones and other mobile transmitters such as personal data assistants with wireless modems.
A problem with known cellular communications systems is that transmission by the base station in one sector of a cell may collide with transmissions by the base station in an adjacent sector of the same cell or in a neighboring cell. For example, orthogonal frequency division multiplexed (OFDM) systems take a given bandwidth and splits it into a number of evenly spaced tones that can be used to transmit data. When transmissions by base stations in overlapping sectors and/or cells use the same tone or set of tones, interference may occur over a period of time due to the operation of base stations in adjacent sectors and/or neighboring cells. This problem is particularly noticeable where transmissions are periodic or nearly periodic.
In periodic or nearly periodic situations, mutual interference caused by base stations in adjacent sectors and/or cells may be highly correlated. For example, when the tones used by a base station A corresponding to a first sector is the same as the tones used by another base station B corresponding to an adjacent sector, in the next transmission period, the tones used by base station A will again be the same as those used by base station B in the case where the tones are assigned using the same function and recur periodically. Correlated interference of this type can cause signals transmitted by the same two base stations to repeatedly interfere with each other over a long period of time. If a wireless terminal is located in the overlapping area between the two base stations, the wireless terminal's receiver may not be able to detect the signals correctly from the downlink signal for a long period of time.
In order to reduce the risk of correlated or prolonged interference it would be beneficial if it was possible to assign tones to the base stations in neighboring sectors and cells in a manner that would minimize the risk of correlated interference.
In view of the above discussion, it becomes apparent that there is a need for minimizing the potential for collisions between transmissions that occur in adjacent cells and neighboring cells of a wireless communications system. It is desirable that the probability that transmissions from any given base station in adjacent sectors or neighboring cells will collide repeatedly be controlled and/or minimized to avoid extended periods where communication signals are blocked for any particular device.